1. Field of the Invention
The field of the invention is a drive wheel slip control system for a vehicle which operates to prevent generation of an excessive slip in a drive wheel.
2. Description of the Prior Art
Conventionally, as such a drive wheel slip control system for a vehicle, of the type having means to reduce torque of the drive wheel when the slippage of the drive wheel becomes excessively large, there has been proposed an arrangement wherein the reference value for obtaining an appropriate slip rate of the drive wheel is changed dependent on the friction coefficient of a road surface which is estimated from a longitudinal acceleration of the vehicle (for example, see Japanese Patent Publication Kokoku No. 20051/83) and an arrangement wherein the yaw movement of the vehicle is corrected when detecting that the yaw movement has been deviated from the steering characteristic desired by a driver of the vehicle (see Japanese Patent Application Publication Kokai No. 219828/88).
However, in case of the former arrangement, the friction coefficient of road surface is merely estimated from the longitudinal acceleration of a vehicle and the estimated friction coefficient is used to change the reference speed for the wheel, so that if the wheel is accelerated during travelling on a high grip road and turning around a limiting lateral force of a tire, for example, the longitudinal acceleration of the vehicle cannot be high and the road surface may erroneously be judged to be a low grip one to select a lower reference value, which undesirably leads to an excessive suppression of the drive wheel slip. In addition, as the limiting lateral force of the tire is originally at a low level when the road grip force is low and particularly in a rear wheel drive vehicle any small variation in the drive force (an increase thereof) easily places the vehicle in a fishtailing condition, it is desired to keep the reference slip value for the vehicle when starting in an accelerated manner at a lower level. However, when the road condition is judged to be a high grip, it is required to raise the reference value promptly; otherwise deterioration in the accelerating performance may occur.
In the latter arrangement, control is effected to lower the reference speed under a specific road surface-tire condition which has been obtained by experiments from a deviation between a reference yaw rate and an actual yaw rate caused during turning of a vehicle which requires a tire to keep its lateral force. Accordingly, there occur such inconveniences under a high grip condition that the drive wheel slip is suppressed excessively to deteriorate the accelerating performance, that a drift travel in a power-on state is disturbed, and that the rise in the drive force after the vehicle has passed a corner is delayed. As a result, it is difficult to perform a sporty travel and utilize the accelerating characteristic inherent to the vehicle.